Genetics plays a fundamental role in shaping various characteristics, including blood type. A child’s unique blood type is determined by the specific combination of genetic information inherited from their parents.
The ABO Blood Group System
Human blood is classified into four main types within the ABO system: A, B, AB, and O. These classifications depend on the presence or absence of specific protein markers, called antigens, found on the surface of red blood cells. Type A blood has A antigens, while Type B blood has B antigens. Individuals with Type AB blood possess both A and B antigens on their red blood cells. Conversely, Type O blood lacks both A and B antigens.
The presence or absence of these antigens is crucial for blood compatibility. For instance, a person with Type A blood also has antibodies against B antigens in their plasma, which would react negatively if exposed to Type B blood. Similarly, Type O blood, lacking both A and B antigens, is considered universally compatible for red blood cell transfusions because it will not trigger an immune response from anti-A or anti-B antibodies in a recipient.
Basic Principles of Inheritance
Genes are segments of DNA that carry instructions for building an organism. Each gene has different versions, called alleles, which account for variations in a trait. For the ABO blood group system, there are three main alleles: A, B, and O.
Humans inherit two alleles for each gene, one from each parent. The interaction of these alleles determines the observable trait, or phenotype. In the ABO system, the A and B alleles are dominant over the O allele. This means if an individual inherits an A allele and an O allele, their blood type will be A because the A allele masks the O allele. Similarly, a B allele will mask an O allele.
The A and B alleles are codominant. When both are inherited, both are fully expressed, resulting in Type AB blood. Only when two O alleles are inherited will an individual have Type O blood, as the O allele is recessive.
The Rh Factor in Blood Typing
Beyond the ABO system, another component of blood typing is the Rh factor, often denoted as Rh-positive (Rh+) or Rh-negative (Rh-). This classification is based on the presence or absence of a specific protein called the RhD antigen on the surface of red blood cells. If the RhD antigen is present, the blood is Rh-positive; if it is absent, the blood is Rh-negative.
The Rh factor follows a dominant-recessive inheritance pattern, separate from the ABO system. The Rh-positive allele is dominant over the Rh-negative allele. An individual will be Rh-positive if they inherit at least one Rh-positive allele. To be Rh-negative, an individual must inherit two Rh-negative alleles, one from each parent.
How Parental Genes Determine a Child’s Blood Type
For the ABO system, each parent contributes one of their two alleles to their offspring. For example, a parent with Type A blood could have two A alleles (AA) or one A and one O allele (AO). Similarly, a Type B parent could be BB or BO. A Type AB parent always contributes either an A or a B allele, and a Type O parent always contributes an O allele.
If one parent contributes an A allele and the other a B allele, the child will have AB blood due to codominance. If both parents contribute an O allele, the child will have Type O blood. Even if both parents have Type A blood (e.g., both are AO), they could still have a child with Type O blood if both pass on their O allele. The Rh factor is inherited similarly, with each parent contributing one Rh allele.
If both parents are Rh-negative, their child will always be Rh-negative, as they can only pass on the recessive Rh-negative allele. However, if both parents are Rh-positive, they could still have an Rh-negative child if both carry and pass on the recessive Rh-negative allele. Conversely, an Rh-positive parent and an Rh-negative parent can have a child who is either Rh-positive or Rh-negative, depending on the Rh-positive parent’s specific genetic makeup.